The present invention relates to coherent imaging systems using vibratory energy, such as ultrasonic or electromagnetic waves, and, more particularly, to novel apparatus for testing of data conversion, data transfer and the like functions in at least one channel of a vibratory energy imaging system.
Methods and apparatus for fully digital beam formation in phase-array coherent imaging systems are now well known; one such system, for use in ultrasonic medical imaging and the like, is described and claimed in U.S. Pat. No. 4,839,652, issued June 13, 1989, assigned to the assignee of the present application and incorporated herein in its entirety by reference. Such an imaging system utilizes a phased array sector scanner (PASS) to rapidly and accurately sweep a formed beam of vibratory energy. The desired beam pointing accuracies are obtained by maintaining an accurate set of phase relationships, which are, in fact, a set of time delays between the various ones of a plurality N of transducer elements of the PASS array. By decoupling the required phase accuracy and time delay accuracy from one another, the signals can be coherently summed with greater accuracy. However, proper beam formation requires that both the necessary time resolution and amplitude resolution be provided in each channel, so that the at least one analog-to-digital converter (ADC, utilized for converting the analog RF signal from each channel transducer, at any instant, into a digital data word for processing) in each channel carry out conversions at a sample frequency of at least two times, and usually four times, the maximum operating imager frequency. In an ultrasonic medical imager utilizing signals of up to 10 MHz, each of the N channels (where N may on the order of 64) requires the use of at least one ADC of 7 or 8 bit output resolution, and operates at a 20 or 40 MHz sampling rate; those skilled in the art will immediately utilize that seven or eight bit ADC resolution is insufficient to provide the at least 70 dB of instantaneous dynamic range required in each channel of the imaging system. A mixed analog/digital system (i.e. an analog time gain control circuit or TGC, compression analog amplifier and the like, utilized with an ADC and a subsequent digital baseband subsystem, including an expansion digital random access memory (RAM) look-up remap stage) allows the instantaneous system dynamic range to be increased to the desired level. We use preselected nonlinear compressive/expansive complementary functions to provide a desired inversion function simultaneously with the ability to remove many other system nonlinearities, and also both to remove certain classes of nonlinearities generated by imperfections in the apparatus of each of the plurality N of channels in the system and correct for channel-to-channel differences. It is now highly desirable to provide apparatus which allows testing of the remapping circuits in each channel of the imaging system.